3-d intraoral surface characterization

ABSTRACT

A method for registering an imaging detector to a surface projects and records a sequence having a first sparse pattern of lines followed by a second sparse pattern of lines. A first subset of positions receives lines from both first and second sparse patterns corresponding to a first label. A second subset of positions receives only lines from the first sparse pattern corresponding to a second label. A third subset of positions receives only lines from the second sparse pattern corresponding to a third label. The first, second, and third labels are decoded and each member element of the first, second, and third subsets of positions registered to the imaging detector according to the decoded labels. One or more dense patterns of lines positionally correlated with registered member elements of the decoded labels are projected and recorded. An image of the surface contour is formed according to the recorded pattern.

TECHNICAL FIELD

The invention relates generally to the field of surface shapecharacterization and more particularly relates to intraoral surfaceimaging and measurement using patterned illumination.

BACKGROUND

The ability to determine the 3D structure of small objects is of valuein a variety of applications, including intra-oral or dental imaging.Intra-oral imaging presents a number of challenges for detecting 3-Dstructure, such as those relating to difficulty in access andpositioning, optical characteristics of teeth and other features withinthe mouth, and the need for precision measurement of irregular surfaces.

A number of techniques have been developed for obtaining surface contourinformation from various types of objects in medical, industrial, andother applications. Optical 3-dimensional (3-D) measurement methodsprovide shape and depth information using images obtained from patternsof light directed onto a surface. Various types of imaging methodsgenerate a series of light patterns and use focus or triangulation todetect changes in surface shape over the illuminated area.

Surface contour imaging uses patterned or structured light andtriangulation to obtain surface contour information for structures ofvarious types. In contour imaging, a pattern of lines or other featuresis projected toward the surface of an object from a given angle. Theprojected pattern from the surface is then viewed from another angle asa contour image, taking advantage of triangulation in order to analyzesurface information based on the appearance of contour lines. Phaseshifting, in which the projected pattern is incrementally spatiallyshifted for obtaining additional measurements at offset locations, istypically applied as part of surface contour imaging, used in order tocomplete the contour mapping of the surface and to increase overallresolution in the contour image.

Surface contour imaging using structured light has been used effectivelyfor solid, highly opaque objects and has been used for characterizingthe surface shape for some portions of the human body and for obtainingdetailed data about skin structure. However, a number of technicalobstacles have prevented effective use of contour projection imaging ofthe tooth. One particular challenge with dental surface imaging relatesto tooth translucency. Translucent or semi-translucent materials ingeneral are known to be particularly troublesome for patterned lightimaging. Subsurface scattering in translucent structures can reduce theoverall signal-to-noise (S/N) ratio and shift the light intensity,causing inaccurate height data. Another problem relates to high levelsof reflection for various tooth surfaces. Highly reflective materials,particularly hollowed reflective structures, can effectively reduce thedynamic range of this type of imaging.

From an optical perspective, the structure of the tooth itself presentsa number of additional challenges for structured light projectionimaging. Teeth can be wet or dry at different times and along differentsurfaces and portions of surfaces. Tooth shape is often irregular, withsharp edges. As noted earlier, teeth interact with light in a complexmanner. Light penetrating beneath the surface of the tooth tends toundergo significant scattering within the translucent tooth material.Moreover, reflection from opaque features beneath the tooth surface canalso occur, adding noise that degrades the sensed signal and thusfurther complicates the task of tooth surface analysis. Not all lightwavelengths can be detected with equal accuracy. Thus, a multi-spectralor multicolor approach can be less satisfactory in some cases.

One corrective measure that has been attempted is application of acoating that changes the reflective characteristics of the tooth surfaceitself. To compensate for problems caused by the relative translucenceof the tooth, a number of conventional tooth contour imaging systemsapply a paint or reflective powder to the tooth surface prior to surfacecontour imaging. This added step enhances the opacity of the tooth andeliminates or reduces the scattered light effects noted earlier.However, there are drawbacks to this type of approach. The step ofapplying a coating powder or liquid adds cost and time to the toothcontour imaging process. Because the thickness of the coating layer isoften non-uniform over the entire tooth surface, measurement errorsreadily result. More importantly, the applied coating, while itfacilitates contour imaging, can tend to mask other problems with thetooth and can thus reduce the overall amount of useful information thatcan be obtained.

Even where a coating or other type of surface conditioning of the toothis used, however, results can be disappointing due to the pronouncedcontours of the tooth surface and inherent difficulties such as angularand space limitations. It can be difficult to provide sufficient amountsof light onto, and sense light reflected back from, all of the toothsurfaces. For example, different surfaces of the same tooth can beoriented at 90 degrees relative to each other, making it difficult todirect enough light for accurately imaging all parts of the tooth.

A number of problems complicate mapping of an illumination array tosensor circuitry for accurate surface contour measurement. Becausemultiple images must be captured with the teeth in the same position,any type of movement of the camera or of the patient can complicate themeasurement task or require re-imaging and additional measurement andcomputation time. Thus, it is advantageous to reduce the number ofimages and amount of time needed for accurate mapping. At the same time,however, measurement improves when multiple images can be obtained andtheir respective data correlated. Given these conflictingconsiderations, it can be seen that there are advantages to moreefficient pixel mapping techniques that obtain a significant amount ofsurface contour data from a small number of images.

SUMMARY

An object of the present disclosure is to advance the art of surfacecontour characterization of teeth and related intraoral structures.Exemplary embodiments of the present disclosure provide 3-D surfaceinformation about a tooth by illuminating the tooth surface with anarrangement of light patterns that help to more closely map pixellocations on a digital imaging array with pixel locations from anillumination device. Advantageously, exemplary embodiments can be usedwith known illumination and imaging component arrangements and isadapted to help reduce ambiguity of sensed patterns when comparedagainst conventional contour detection methods. As a further advantage,exemplary embodiments of the present disclosure require fewer frames ofillumination than other methods, alleviating the problems caused bypatient or operator motion.

These objects are given only by way of illustrative example, and suchobjects may be exemplary of one or more embodiments of the disclosure.Other desirable objectives and advantages inherently achieved by thedisclosed method may occur or become apparent to those skilled in theart. The invention is defined by the appended claims.

According to one aspect of the disclosure, there is provided a methodfor registering an imaging detector to a surface, the method executed atleast in part on a computer and can include projecting and recording, ona portion of the surface, a sequence comprising a first sparse patternof lines followed by a second sparse pattern of lines, wherein a firstsubset of positions on the surface is illuminated by lines from bothfirst and second sparse patterns corresponding to a first label, asecond subset of positions on the surface is illuminated only by linesfrom the first sparse pattern corresponding to a second label; and athird subset of positions on the surface is illuminated only by linesfrom the second sparse pattern corresponding to a third label; decodingthe first, second, and third labels from the surface positions andregistering each member element of the first, second, and third subsetsof positions to the imaging detector according to the decoded labels;projecting and recording one or more dense patterns of lines that arepositionally correlated with registered member elements of the decodedlabels; and forming and displaying an image of the surface contouraccording to the recorded patterns.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of thedisclosure will be apparent from the following more particulardescription of the embodiments of the disclosure, as illustrated in theaccompanying drawings. The elements of the drawings are not necessarilyto scale relative to each other.

FIG. 1 is a schematic diagram that shows mapping a sensor pixel array toan illumination array according to a surface.

FIG. 2A shows illumination of a tooth surface with a single line oflight.

FIG. 2B shows illumination of a tooth surface with multiple lines oflight.

FIG. 3 is a schematic diagram showing an imaging apparatus.

FIG. 4 is a schematic diagram that shows sparse and fully populatedimage frames for contour characterization.

FIG. 5 is a plan view of an exemplary multiline image.

FIG. 6 is a logic flow diagram that shows a sequence for obtainingsurface contour image data according to an embodiment of the presentdisclosure.

FIGS. 7A, 7B, and 7C are schematic views that show exemplary sequencesof sparse and fully populated frames that can be projected onto thesurface for surface characterization according to embodiments of thepresent disclosure.

FIG. 8A shows schematically a portion of a single line of illuminationpixels from the illumination array, energized for forming a portion of afully populated multiline image frame.

FIG. 8B shows schematically a portion of a single line of illuminationpixels from the illumination array, energized for forming a portion of asparse image frame.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following is a description of exemplary embodiments, reference beingmade to the drawings in which the same reference numerals identify thesame elements of structure in each of the several figures.

Where they are used, the terms “first”, “second”, and so on, do notnecessarily denote any ordinal, sequential, or priority relation, butmay be used for more clearly distinguishing one element, set ofelements, or time interval from another. The term “exemplary” indicatesthat the description is used as an example, rather than implying that itis an ideal.

In the context of the present disclosure, the terms “viewer”,“operator”, and “user” are considered to be equivalent and refer to theviewing practitioner or technician or other person who views andmanipulates an image, such as a dental image, on a display monitor.

The term “set”, as used herein, refers to a non-empty set, as theconcept of a collection of elements or members of a set is widelyunderstood in elementary mathematics. The term “subset”, unlessotherwise explicitly stated, is used herein to refer to a non-emptysubset, that is, to a subset of the larger set, having one or moremembers. For a set S, a subset may comprise the complete set S,including all members. A “proper subset” of set S is strictly containedin set S and excludes at least one member of set S. In the context ofthe present disclosure, the term “subset” is used to mean a non-emptyproper subset, unless otherwise specifically noted.

The schematic diagram of FIG. 1 shows, with the example of a single lineof light L, how patterned light is used for obtaining surface contourinformation. A mapping is obtained as an illumination array 10 projectsor directs a pattern of light onto a surface 20 and a correspondingimage of a reflected line L′ is formed on an imaging sensor array 30.Each pixel 32 on imaging sensor array 30 can map to a correspondingpixel 12 on illumination array 10 according to modulation by surface 20.Shifts in pixel position, as represented in FIG. 1, yield usefulinformation about the contour of surface 20. It can be appreciated thatthe basic pattern shown in FIG. 1 can be implemented in a number ofways, using a variety of illumination sources including multipleidentical or different illumination sources and sequences for projectingthe images and using one or more different types of sensor arrays 30 forobtaining or acquiring image data. Illumination array 10 can utilize anyof a number of types of arrays used for light modulation, such as aliquid crystal array or digital micromirror array, such as that providedusing the Digital Light Processor or DLP device from Texas Instruments,Dallas, Tex. This type of spatial light modulator is used in theillumination path to change the projected light pattern as needed forthe mapping sequence.

FIGS. 2A and 2B show aspects of one problem with conventional approachesfor using patterned light to obtain surface structure information fromthe human tooth. FIG. 2A shows illumination with a single line of light14 onto the tooth, with pronounced shifting of the illumination at thetooth edges. Projection of a single line in this manner, scanned acrossthe tooth and imaged at numerous points during the scan, can provideaccurate information about portions of the surface area; however, someinformation is lost even with this method, such as where line segmentsare separated from each other. FIG. 2B shows surface imaging using apattern with multiple lines of light. Where there are abrupt transitionsalong the surface, it can be difficult to positively identify thesegments that correspond to each projected line and mismatches caneasily occur, leading to inaccurate conclusions about surfacecharacteristics. For example, it can be difficult to determine whetherline segment 16 is from the same line of illumination as line segment 18or adjacent line segment 24.

Exemplary method and/or apparatus embodiments can address problems ofsurface contour mapping using a sequence of projected images or imageframes that help to better correlate pixels on the imaging sensor arraywith projected lines from the illumination array and to provide thiscorrelation using a relatively small number of projected images. Inorder to improve the efficiency, accuracy, and/or speed of the contourimaging process, exemplary encoding method and/or apparatus embodimentscan employ a combination of sparse representation and dense contourmeasurement techniques that provide an inherent check on detectionaccuracy and a highly efficient registration of the illuminationdevice(s) and sensing device(s) to the surface.

The schematic diagram of FIG. 3 shows an imaging apparatus 70 forprojecting and capturing both sparse images 52 a and 52 b that providean encoding and register the imaging apparatus and one or more densemultiline images that map the surface contour. A control logic processor80, or other type of computer controls the operation of illuminationarray 10 and imaging sensor array 30. Image data from surface 20, suchas from a tooth 22, is obtained from imaging sensor array 30 and storedin a memory 72. Control logic processor 80 processes the received imagedata and stores the mapping in memory 72. The resulting image frommemory 72 is then optionally displayed on a display 74. Memory 72 mayalso include a display buffer. An optional motion sensor 60, such as anaccelerometer, can enable detection of motion during illuminationprojection.

Certain exemplary method and/or imaging apparatus embodiments forsurface contour characterization address various problems of projectingand detecting a pattern of light with repeated lines or other patternedillumination features wherein the center coordinates of the repeatedillumination feature are clearly identified and registered and whereinthe source of each illumination feature is identified. The particulargeometry of the teeth greatly complicates this task. Some projectedstripes or other projected illumination features may not be perceptibleat a particular camera angle or are broken into fragments that must beaccounted for. Each projected stripe can have an identifying tag orlabel; however, the number of possible labels is limited. The positionof any given projected line of light from the center of the depth rangeis known, along with data on how far the stripe can move at the end ofits depth range. Two stripes that are farther apart than this determinedor maximum distance movement are no longer unique.

Encoding Scheme

In order to provide a labeling scheme that can address and/or eliminateambiguity in mapping or correlating a illumination feature detected bythe imaging sensor array to a corresponding location on the illuminationarray, some exemplary embodiments of the present disclosure provide anencoding that associates a symbol or label to each line and aline-to-line encoding pattern that can be readily decoded and checkedand that has sufficient pattern length so that repetition of labels doesnot affect illumination feature identification.

One exemplary embodiment of the present disclosure uses a binaryencoding that allows representation of a pattern, such as a deBruijnsequence, known to those skilled in combinatorial mathematics. AdeBruijn sequence is cyclic and uses a set of labels or elements of sizeor cardinality m, wherein m is an integer. A deBruijn sequence uses mlabels or elements to create n>m unique positions. For the set ofelements or labels {R, G, B}, an example of length 12 is as follows:

RGBRGRBRBGBG

There are four “B” labels or symbols in this sequence, but there is onlyone instance of a B with a G on the left and an R on the right. Bycombining three adjacent symbols, unique positions in the sequence areidentified.

deBruijn sequences are usually designed to be cyclic, so that theuniqueness is maintained even when the sequence is repeated. Forexample, considering the 24 element sequence:

RGBRGRBRBGBGRGBRGRBRBGBG

The triads BGR and GRG, which occur on the boundary of the two identicaldeBruijn sequences, are unique. One still needs a way to distinguish thefirst and second occurrence of the 12-element sequence, but there is noconfusion as to the boundary. The general deBruijn sequence has msymbols and a length n, with groups of g symbols or labels used toidentify positions. The baseline description given herein uses valuesm=3, n=12, g=3.

A monochrome dental imaging system (e.g., monochrome sensor) cannotencode a deBruijn sequence using features in a single frame becausethere is only one detectable state of illumination available, allowingonly a binary encoding. To provide the needed additional dimension, atime sequence is used to expand the encoding. By using an orderedsequence of frames, it is possible to encode a set of m=3 elements orlabels, such as using as few as two successive image frames, employing asparse frame representation. For the example sparse frame representationshown with reference to FIG. 4, the following example encoding is used,with three labels:

1, 0 - R 0, 1 - G 1, 1 - B

With this encoding sequence, the two sparse image frames are sufficientto enable encoding for three symbols. It should be noted that additionalsparse image frames can be used for encoding an expanded number ofsymbols or labels. For example, three sparse image frames would enableencoding the use of a set of as many as (2³−1)=7 symbols, such as:

{R, G, B, C, M, Y, K}.

For the sparse encoding scheme used in a plurality of exemplaryembodiments of the present disclosure, none of the elements are definedfor a 0, 0 (or 0, 0, 0) encoding. For each spaced encoding position, thesequence of two or more sparse illumination frames must project at leastone line of light or other illumination feature.

By way of example, FIG. 4 shows a sequence of two sparse frames F1 andF2 that are projected in series to provide the example encodingRGBRGRBRBR, with the elements or labels R, G, B encoded in position asshown above. Dashed lines indicate omitted lines or illuminationfeatures that are not projected as part of the sparse frame, providing abinary “0”; unbroken lines indicate a “1”. For each position, there mustbe a “1” in at least one of the first or second sparse frames F1 and F2.More generally stated, in a set of sparse image frames F1, F2 that areused for this encoding, each line or other illumination feature used inthe encoding must appear at least once.

Frames F1 and F2 are sparse frames because they do not contain a line orfeature in every available position, that is, at every unit increment,as does a fully populated multiline frame F (also termed a dense framein the present disclosure). The spacing increment between next adjacentillumination features in each of frames F1 and F2 can vary by an integermultiple of a standard spacing unit, so that space between next adjacentlines, for example, is one, two, three, or more standard unitincrements.

In FIG. 4, by way of illustration, sparse image frames F1 and F2 onlyshow line patterns encoding a 12 element sequence. Generally, the set ofline patterns are repeated a number of times horizontally across eachimage for complete identification of all the line groups in illuminationarray 10. The lines in the dense frame F are correspondingly repeatedhorizontally across the whole illumination array 10, with a linedisposed at each unit increment position.

In terms of projected lines or other pattern elements, each of sparseframes F1 and F2 used for mapping can be considered as a proper subsetof a fully populated multiline frame F. That is, each sparse frame F1omits at least one element or illumination pattern feature of fullypopulated, dense frame F. Similarly, each sparse frame F2 omits at leastone element of fully populated frame F. The union of sparse frames F1and F2 includes all elements of fully populated or dense frame F.

With respect to the image surface that is sensed by the detector array,a first subset of positions on the surface receive illumination featuresfrom both first and second illumination patterns that encode a firstsymbol, a second subset of positions on the surface receives onlyillumination features from the first illumination pattern that encode asecond symbol; and a third subset of positions on the surface receivesonly illumination features from the second illumination pattern thatencode a third symbol. This type of encoding provides a useful mappingof surface locations with illumination and detector pixels, as describedwith reference to FIG. 1.

FIG. 5 is a plan view of an exemplary fully populated multiline image54. Illumination features 84 are spaced at equal intervals, with thestandard distance unit or unit increment between them shown as adistance d. Illumination features 84 are preferably 1 pixel wide,however, in one embodiment, 2 or more pixels can be used forillumination features 84 or in a sparse or dense frame. It should benoted that a pattern of lines provides one useful type of illuminationfeature for surface characterization, advantaged because the shift of adetected line can be readily used to determine surface featuring.However, alternate illumination features can be utilized as anillumination pattern, such as curved lines, dots, aligned shapes, orother suitable pattern.

Specifying an Encoding

While the deBruijn encoding scheme described previously can have anumber of advantages for providing unique encoding that can provide anextended non-repeating sequence, other encoding sequences can be usedwith the sparse frame projection described with reference to FIG. 4. Inan exemplary embodiment, it may be considered advantageous to havevarious types of repeated patterns, for example, as a check on validreading and interpretation of sensed data.

Sequence for Surface Characterization

In certain exemplary method and/or imaging apparatus embodiments,encoding is used to help register the imaging apparatus of FIG. 3 to thesurface features in the field of view. Given this precise registration,one or more subsequent fully populated frames can then be projected inorder to acquire more fully detailed contour information.

The logic flow diagram of FIG. 6 shows an example sequence for surfacecharacterization that can use the sparse and fully populated framesdescribed with reference to FIG. 4 and can be used in the intraoralimaging apparatus 70 of FIG. 3.

In first and second projection steps S100 and S110 respectively, thesparse frames F1 and F2 are projected onto the surface and detected byimaging sensor array 30 (FIG. 3). Sparse frames F1 and F2 provide apreset or the minimum number of frames needed for a mapping with uniqueidentification of the illumination lines. If subsequent reconstructionstep S170 is carried out on the data from sparse frames F1 and F2, acoarse contour of the surface can be mapped out, albeit at a relativelylow resolution.

Referring again to the FIG. 6 sequence, after projection of the firstand second sparse frames, a first fully populated or dense frame can beprojected in an optional projection step S140. The lines in this denseframe can be shifted by a fraction of the unit increment, such as byhalf of the line pitch, relative to the positions of lines in the sparseframes. When this dense frame and the first and second sparse frames areprocessed together in a reconstruction step S170, they can then generatethe surface contour at double the resolution as would be generated usingonly the first and second sparse frames without the dense frame.

In typical applications and depending in part on the needed resolution,additional fully populated frames can be projected toward the surface inan optional projection step S150. Where additional fully populatedframes are used, the second and subsequent projected frames can bepreferably positionally offset from the initial fully populated frame ofstep S140 and from each other to generate added surface contentinformation. For example, a first optional populated multiline imageframe F can be offset by +0.33d, with a second offset by −0.33d. In thisway, significant gains in image resolution can be obtained with eachadditional fully populated frame F projection. In one exemplaryembodiment, added surface content information can be obtained byinterpolation for positions between fully populated frames or multilineframes. A reconstruction and display step S170 reconstructs and displaysthe computed surface contour that is generated from this processingsequence.

The schematic views of FIGS. 7A, 7B, and 7C show exemplary sequences ofsparse and fully populated frames that can be projected onto the surfacefor surface characterization according to an embodiment of the presentdisclosure. Each sequence proceeds from the top downward in thedescription that follows; however, it should be noted that the sparseframes and dense or fully populated frames can be projected in anysuitable order. Shown in FIG. 7A, sparse frames F1 and F2 are projectedas the minimum frame sequence for surface contour mapping at sparseresolution. This sequence shows that contour characterization can beobtained using as few as two sparse frames.

FIG. 7B shows the frames sequence with the addition of a fully populatedmultiline frame F. In the dense frame F, the dense lines are shifted by0.5d with respect to the line positions of sparse frames F1 and F2.Here, the three-frame sequence shown provides surface contour mapping attwice the resolution of the two frame sequence in FIG. 7A.

FIG. 7C shows an exemplary sequence with the use of two additional fullypopulated multiline frames F and F. The dense, multiline frames F and F′are projected with lines offset by distances +0.33d and −0.33d,respectively, from line positions of sparse frames F1 and F2. This4-frame sequence provides surface contour mapping at 3× the resolutionof the two frame sequence of FIG. 7A. Additional offset fully populatedframes can also be projected, at suitable offsets (e.g., ⅕, ¼, etc. forthe group of dense frames) to provide additional resolution.

It should be noted that the image frames as shown in FIGS. 7A-7C can bedirected to surface 20 in any order, such as sending the fully populatedmultiline image frame(s) first, followed by sparse images. However,there are advantages to providing the sparse images F1 and F2 first toreduce or minimize the effects of hand motion.

In any sequence of frames F1, F2, and F that is used, there can beadvantages to repeating one or more frames. Thus, for example, it can beadvantageous to repeat the first frame that was projected as a finalframe in the sequence. This arrangement allows the system logic toverify that camera position has not shifted relative to the patient, sothat contour information can be verified for accuracy. By way ofexample, this method would repeat frame F1 at the end of the sequenceshown in FIG. 7C, testing the positional data in order to verify thatexcessive shifting of position has not occurred.

By way of illustration, FIG. 8A shows schematically a portion of asingle row of illumination pixels 34 from illumination array 10,energized for forming a portion of a fully populated multiline imageframe F. Shaded pixels 34 in this figure are energized to provideillumination. Multiple rows of pixels are used to form the completeimage; only a single row is shown in FIG. 8A. In a parallelrepresentation, FIG. 8B shows a portion of a single row of illuminationpixels 34 from illumination array 10 that is energized for forming asparse image frame F1. In sparse image frames (e.g., F1, F2), the samepreset or minimum pixel spacing is used, but some of the potentialpixels are de-energized and do not deliver light to surface 20.

Light intensity for each type of image can be the same; however, therecan be advantages to changing intensity for different image types.Suitable adjustment of intensity, where available, can help to reducethe impact of scattered light, for example.

The pattern arrangement shown for lines or other features in the presentembodiment presents regularly spaced lines or other features. However,it should be noted that there can be advantages in providing a densepattern that has an uneven distribution of projected features. Thus, forexample, lines can be more tightly spaced over parts of the surface.Where features are not evenly distributed, with equal unit spacing,sparse frames F1 and F2 are arranged accordingly, so that spatialregistration of illumination features between sparse and dense or fullypopulated frames is maintained. Dense or fully populated frames may omitone or more features found in the sparse frames.

Advantageously, exemplary method and/or apparatus embodiments of thepresent disclosure allow accurate contour characterization using as fewas two, three, or four frames. This contrasts with conventional dentalstructured light techniques that require five or more individual framesof light patterns in order to provide accurate surface characterizationof teeth from a single scanner position. Use of various exemplaryembodiments of the present disclosure allow surface imaging content tobe quickly acquired. At a coarser resolution, surface imaging data canbe acquired using as few as two sparse frames.

Consistent with exemplary embodiments herein, a computer program can usestored instructions that perform on image data that is accessed from anelectronic memory. As can be appreciated by those skilled in the imageprocessing arts, a computer program for operating the imaging system andprobe and acquiring image data in exemplary embodiments of theapplication can be utilized by a suitable, general-purpose computersystem operating as control logic processors as described herein, suchas a personal computer or workstation. However, many other types ofcomputer systems can be used to execute the computer program of thepresent invention, including an arrangement of networked processors, forexample. The computer program for performing exemplary methodembodiments may be stored in a computer readable storage medium. Thismedium may include, for example; magnetic storage media such as amagnetic disk such as a hard drive or removable device or magnetic tape;optical storage media such as an optical disc, optical tape, or machinereadable optical encoding; solid state electronic storage devices suchas random access memory (RAM), or read only memory (ROM); or any otherphysical device or medium employed to store a computer program. Computerprograms for performing exemplary method embodiments may also be storedon computer readable storage medium that is connected to the imageprocessor by way of the internet or other network or communicationmedium. Those skilled in the art will further readily recognize that theequivalent of such a computer program product may also be constructed inhardware.

It should be noted that the term “memory”, equivalent to“computer-accessible memory” in the context of the application, canrefer to any type of temporary or more enduring data storage workspaceused for storing and operating upon image data and accessible to acomputer system, including a database, for example. The memory could benon-volatile, using, for example, a long-term storage medium such asmagnetic or optical storage. Alternately, the memory could be of a morevolatile nature, using an electronic circuit, such as random-accessmemory (RAM) that is used as a temporary buffer or workspace by amicroprocessor or other control logic processor device. Display data,for example, is typically stored in a temporary storage buffer that isdirectly associated with a display device and is periodically refreshedas needed in order to provide displayed data. This temporary storagebuffer is also considered to be a type of memory, as the term is used inthe application. Memory is also used as the data workspace for executingand storing intermediate and final results of calculations and otherprocessing. Computer-accessible memory can be volatile, non-volatile, ora hybrid combination of volatile and non-volatile types.

It will be understood that computer program products of the applicationmay make use of various image manipulation algorithms and processes thatare well known. It will be further understood that computer programproduct exemplary embodiments of the application may embody algorithmsand processes not specifically shown or described herein that are usefulfor implementation. Such algorithms and processes may includeconventional utilities that are within the ordinary skill of the imageprocessing arts. Additional aspects of such algorithms and systems, andhardware and/or software for producing and otherwise processing theimages or co-operating with the computer program product exemplaryembodiments of the application, are not specifically shown or describedherein and may be selected from such algorithms, systems, hardware,components and elements known in the art.

Certain exemplary dental method and/or apparatus embodiments accordingto the application can allow accurate dentition contour characterizationusing as few as two, three, or four frames of structured light. Inexemplary embodiments, a sequence of symbols can be encoded in twosparse frames that are used with a dense uncoded frame, which is shiftedby a pixel fraction pitch from the sparse frames. Although embodimentsof the present disclosure are illustrated using dental imagingapparatus, similar principles can be applied for other types ofdiagnostic imaging and for other anatomy. Exemplary embodimentsaccording to the application can include various features describedherein (individually or in combination).

While the invention has been illustrated with respect to one or moreimplementations, alterations and/or modifications can be made to theillustrated examples without departing from the spirit and scope of theappended claims. In addition, while a particular feature of theinvention can have been disclosed with respect to only one of severalimplementations/embodiments, such feature can be combined with one ormore other features of the other implementations/embodiments as can bedesired and advantageous for any given or particular function. The term“at least one of” is used to mean one or more of the listed items can beselected. The term “about” indicates that the value listed can besomewhat altered, as long as the alteration does not result innonconformance of the process or structure to the illustratedembodiment. Finally, “exemplary” indicates the description is used as anexample, rather than implying that it is an ideal. Other embodiments ofthe invention will be apparent to those skilled in the art fromconsideration of the specification and practice of the inventiondisclosed herein. It is intended that the specification and examples beconsidered as exemplary only, with a true scope and spirit of theinvention being indicated by at least the following claims.

1. A method for registering an imaging detector to a surface, the method executed at least in part on a computer and comprising: projecting and recording, on a portion of the surface, a sequence comprising a first sparse pattern of lines followed by a second sparse pattern of lines, wherein a first subset of positions on the surface is illuminated by lines from both first and second sparse patterns corresponding to a first label, a second subset of positions on the surface is illuminated only by lines from the first sparse pattern corresponding to a second label; and a third subset of positions on the surface is illuminated only by lines from the second sparse pattern corresponding to a third label; decoding the first, second, and third labels from the surface positions and registering each member element of the first, second, and third subsets of positions to the imaging detector according to the decoded labels; projecting and recording one or more dense patterns of lines that are positionally correlated with registered member elements of the decoded labels; and forming and displaying an image of the surface contour according to the recorded patterns.
 2. The method of claim 1 further comprising verifying the decoding according to a predetermined pattern of labels and repeating the projection of at least one of the first and second sparse patterns if the verification fails.
 3. The method of claim 1 wherein the labels are arranged as a deBruijn sequence.
 4. The method of claim 1 further comprising detecting motion during pattern projection.
 5. The method of claim 1 wherein projecting and recording is performed using a hand-held imaging apparatus.
 6. The method of claim 1 further comprising storing or transmitting the surface contour image.
 7. The method of claim 1 wherein the lines in the dense patterns are evenly spaced.
 8. The method of claim 1 further comprising repeating the projection of either the first sparse pattern or the second sparse pattern of lines and verifying the decoding of one or more of the first, second, and third labels.
 9. The method of claim 1 further comprising repeating the projection of one or more dense patterns of lines and verifying the positional correlation with registered member elements of the decoded labels.
 10. The method of claim 1 wherein one or more of the projected dense patterns are positionally offset from the sparse pattern of lines by half of the shortest incremental distance between lines in the sparse pattern.
 11. A method for registering an imaging detector to a surface, the method executed at least in part on a computer and comprising: projecting, onto a first portion of the surface, a first mapping image frame comprising a first pattern of illumination features followed by second mapping image frame of illumination features comprising a second pattern of illumination features, wherein a first subset of positions on the surface receives illumination features from both first and second illumination patterns that encode a first label, a second subset of positions on the surface receives only illumination features from the first illumination pattern that encode a second label, and a third subset of positions on the surface receives only illumination features from the second illumination pattern that encode a third label; decoding the first, second, and third labels from the surface and registering each member clement of the first, second, and third subsets of positions to the imaging detector according to the decoded labels; projecting and recording, onto the first portion of the surface, a third image frame having a third illumination pattern of spaced-apart features that are correlated with the first and second illumination patterns and the decoded labels; and forming and displaying an image of the surface contour according to the recorded illumination patterns from the first, second, and third image frames.
 12. The method of claim 11 wherein the spaced-apart features are evenly spaced.
 13. The method of claim 12 further comprising projecting the third image frame at a position that is offset from the first portion of the surface by less than the distance between the evenly spaced features of the third image frame.
 14. The method of claim 11 further comprising verifying the decoding before projecting the third image frame.
 15. The method of claim 11 further comprising re-projecting either the first or second mapping image frame after projecting the third image frame.
 16. A dental intraoral imaging apparatus, comprising: encoding means for defining a set of three symbols and a sparse encoding of each symbol according to an two interval time sequence that represents each symbol at a location by projecting a line of light onto the same location from 1 to 2 times; means for correlating a pixel on an illumination pixel array to a corresponding pixel on a sensor array by sequentially projecting and recording light directed from the illumination array pixel to the sensor array pixel as part of an illumination sequence that encodes a series combination of the set of symbols in two successive sparse frames of light, means for characterizing the surface by projecting and recording one or more dense frames of spaced lines of light from the illumination pixel array, wherein the dense frames are positionally registered to the sparse frames and each dense frame includes lines of light not in at least one of the sparse frames of light; and means for forming an image of the surface contour according to the recorded patterns from the two successive sparse frames and from the one or more dense frames.
 17. The dental intraoral imaging apparatus of claim 16 wherein spacing between a first line of light and a second line of light that is next adjacent to the first line of light in the sparse frames is two or more times the spacing between the second line of light and a third line of light that is next adjacent to the second line, wherein the dense frames of spaced lines of light from the illumination pixel array are equally spaced. 